The present invention relates generally to a superconductive magnet (such as, but not limited to, a helium-cooled and/or cryocooler-cooled superconductive magnet) used to generate a high magnetic field as part of a magnetic resonance imaging (MRI) system, and more particularly to such a magnet having an open design and having a uniform (i.e., homogeneous) magnetic field within its imaging volume.
MRI systems employing superconductive or other type magnets are used in various fields such as medical diagnostics. Known superconductive magnets include liquid-helium cooled and cryocooler-cooled superconductive magnets. Typically, for a helium-cooled magnet, the superconductive coil assembly includes a superconductive main coil which is at least partially immersed in liquid helium contained in a helium dewar which is surrounded by a dual thermal shield which is surrounded by a torroidal-shaped vacuum enclosure having a bore and a longitudinal axis. In a conventional cryocooler-cooled magnet, the superconductive main coil is surrounded by a thermal shield which is surrounded by a torroidal-shaped vacuum enclosure having a bore and a longitudinal axis, and the cryocooler coldhead is externally mounted to the vacuum enclosure with the coldhead's first stage in thermal contact with the thermal shield and with the coldhead's second stage in thermal contact with the superconductive main coil. Nb-Ti superconductive coils typically operate at a temperature of generally 4 Kelvin, and Nb-Sn superconductive coils typically operate at a temperature of generally 10 Kelvin.
A superconductive coil assembly of a conventional MRI system includes three pulsed (i.e., not time-constant) resistive gradient coils including a Z-axis coil which is coaxially aligned with the longitudinal axis and which carries a pulsed electric current which may be in a direction opposite to the current direction of the superconductive main coils. Such gradient coils are all located outside the vacuum enclosure (i.e., coil housing) in the bore. The superconductive coil assembly also includes several resistive radio-frequency coils all located outside the vacuum enclosure (i.e., coil housing) in the bore.
Known superconductive magnet designs include closed magnets and open magnets. Closed magnets typically have a single, tubular-shaped superconductive coil assembly having a bore and a longitudinal axis. The superconductive coil assembly includes several radially-aligned and longitudinally spaced-apart superconductive main coils each carrying a large, identical electric current in the same direction. The superconductive main coils are thus designed to create a magnetic field of high uniformity within a spherical imaging volume centered within the magnet's bore where the object to be imaged is placed. Although the magnet is so designed to have a highly uniform magnetic field within the imaging volume, manufacturing tolerances in the magnet and magnetic field disturbances caused by the environment at the field site of the magnet usually require that the magnet be corrected at the field site for such minor irregularities in the magnetic field. Typically, the magnet is shimmed at the field site by using pieces of iron, or, for Nb-Ti superconductive magnets cooled by liquid helium, by using numerous Nb-Ti superconductive correction coils. The correction coils are placed within the superconductive coil assembly typically radially near and radially inward of the main coils. Each correction coil carries a different, but low, electric current in any required direction including a direction opposite to the direction of the electric current carried in the main coils. It is also known to shim a closed magnet by using numerous resistive DC shim coils all located outside the vacuum enclosure (i.e., coil housing) in the bore. The resistive DC shim coils each produce time-constant magnetic fields and may include a shim coil coaxially aligned with the longitudinal axis and carrying an electric current in a direction opposite to the current direction of the superconductive main coils to correct a harmonic of symmetrical inhomogeneity in the magnetic field within the imaging volume caused by manufacturing tolerances and/or site disturbances.
Open magnets typically employ two spaced-apart superconductive coil assemblies with the space between the assemblies allowing for access by medical personnel for surgery or other medical procedures during MRI imaging. The patient may be positioned in that space or also in the bore of the toroidal-shaped coil assemblies. The open space helps the patient overcome any feelings of claustrophobia that may be experienced in a closed magnet design. The literature is largely silent on how superconductive open magnets can be made to have a magnetic field of high uniformity within the imaging volume when the creation of the open space between the superconductive coil assemblies grossly distorts the magnetic field creating a magnetic field of low uniformity within the imaging volume.
What is needed is an open MRI magnet designed to have a highly uniform magnetic field within its imaging volume despite the gross magnetic field distortions created by the open space between the magnet's superconductive coil assemblies.